Gas turbine engines, turbine parts and the like which operate at high temperatures well above 650.degree. C., for example, 1100.degree.-1600.degree. C., require materials exhibiting unusual oxidation resistance and good strength. At the present time, nickel-base superalloys are the most widely used materials in aircraft engines, since they can withstand temperatures up to 1100.degree. C. In order to extend the use temperature to 1600.degree. C. or higher, increase efficiency, and reduce fuel costs, advanced ceramics and refractory metals have been considered. New materials such as ceramic matrix composites show some potential in terms of thermal capability and strength/weight ratio; however, they also present high risks in terms of reliability. Refractory metals or intermetallic compounds offer another possibility for high temperature matrix materials.
In particular, niobium and niobium alloys among the refractory metals have been considered for use because of their favorable combination of density, high melting temperature, cost and availability. However, niobium-base alloys oxidize very rapidly above 650.degree. C. Also, they are embrittled by oxygen, carbon and nitrogen. While niobium alloys can be coated with an oxidation resistant silicide, such as MoSi.sub.2, coating performance and reliability are not satisfactory for advanced gas turbines which are required to have extended lives at high temperatures.
The oxidation behavior of niobium base alloys has been the subject of considerable research in the past. It has been shown that the slowest oxidation rate of all niobium-aluminum compounds was observed for NbA.sub.3. The parabolic oxidation constant, k.sub.p, was found to be two orders of magnitude higher than that of NiAl which forms a protective aluminum scale at 1200.degree. C. An alumina inner layer is formed on NbAl.sub.3 adjacent to the metal-oxide interface while an NbAlO4 outer layer is formed at the oxide-gas interface. More recent work has shown the feasibility of forming compact, adherent alumina scales on niobium-aluminum alloys at greatly reduced aluminum contents, but at and above 1400.degree. C. These modified alloys have included additions of titanium to increase the solubility and diffusivity of aluminum, and chromium and/or vanadium to decrease the solubility-diffusivity product of elemental oxygen in the alloy.
U.S. Pat. No. 2,838,396 discloses modified niobium-aluminum-chromium alloys asserted to have high strength and oxidation resistance at temperatures ranging from 1000-1300.degree. C. and higher. The disclosed alloys have a reduced aluminum content of 1-20%, and include chromium in a range 1-30%. In addition to chromium, the alloys optionally include one or more of the elements cobalt, nickel, tungsten and zirconium, from 1-5% by weight of one or more of the elements beryllium, manganese, silicon, thorium and vanadium, and 0-2% by weight of one or more of the elements boron, carbon, calcium and cerium to impart certain desired characteristics, such as the properties of protective oxide scale or a special metallurgical response to heat treatment or fabrication, etc.